Last month's botched landing of a Russian capsule returning from the international space station was caused by the failure of an equipment module to separate from the capsule on time, a Russian space official said Wednesday.

The Soyuz TMA-11 craft carrying U.S. astronaut Peggy Whitson, Russian cosmonaut Yuri Malenchenko and South Korean bioengineer Yi So-yeon landed hundreds of miles off course when it bounced onto the steppes of northern Kazakhstan April 11.

The three were subjected to severe G-forces, communications were disrupted and Russian officials said they had been in serious danger during the descent.

Alexei Krasnov, who heads Russia's manned space program, said after the Soyuz's separation from the space station, the equipment bay module was supposed to detach, allowing the capsule to enter the atmosphere and descend to Earth smoothly.

That did not happen, he said, and the Soyuz went into a "ballistic" descent.

He said Russian experts would soon finish the final report on the flawed landing, which was the second in a row -- and the third since 2003.

Officials with the U.S. space agency, NASA, have been watching the progress of the investigation closely since Russian-built Soyuz and Progress ships are a primary means for getting crew and cargo to and from the orbiting station.

As well, a Soyuz capsule is always docked at the station as an emergency "lifeboat" in case the crew needs it.

NASA officials on Monday set a May 31 launch date for the next space shuttle mission and said that Russia's investigation into last month's rocky landing of its own spacecraft should not interfere.

Bill Gerstenmaier, NASA's space operations chief, said there would be no benefit to delaying the shuttle mission to allow the Russians more time to investigate the landing.

An American is scheduled to blast into orbit aboard the next Soyuz in October.